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Abstract: A two-step route comprising treatment in sodium aluminate followed by acid washing was developed to introduce secondary mesoporosity in ZSM-5 zeolites with different Si/Al ratios. The parent and treated zeolites were characterized by ICP-OES (solids and filtrates), XRD, TGA, TEM, gas adsorption (N2 at 77K and Ar at 87K), FTIR, and NH3-TPD. The contact of the zeolite with NaAlO2 solutions induced mild leaching of framework silicon, viz. desilication. However, the porosity of the resulting samples was mostly blocked by NaAlO2-derived deposits and Si-containing debris. A subsequent acid washing with HCl or oxalic acid removed the blocking species, largely regaining the native microporosity and further uncovering the extensive intracrystalline mesoporosity generated in the first step. In order to maximize the micro- and mesoporosity in the final zeolites, the conditions of the NaAlO2 and acid treatments were varied with respect to concentration, temperature, and time. The efficiency of the two-step treatment was remarkable for zeolites with Si/Al=25 and 40. The mesopore surface area of the hierarchical ZSM-5 was 3–4 times higher than in the parent zeolites, while other intrinsic properties like crystallinity and acidity were largely preserved. The physico-chemical properties of hierarchical ZSM-5 obtained by the two-step route and by conventional desilication in NaOH were compared. Both desilication variants led to equivalent mesopore surface areas, although the two-step route presented a higher solid yield and produced smaller mesopores due to the much lower amount of silicon leached compared to the treatment in sodium hydroxide. Mechanistic and kinetic aspects of silicon dissolution and porosity development in ZSM-5 were discussed on the basis of alkaline treatments in mixtures of NaOH and NaAlO2. [Copyright &y& Elsevier]

Abstract: Remarkable progress towards implementation of catalytic technology for N2O mitigation in nitric acid plants has been experienced in recent years. EnviNOx®, one of the commercial processes developed by Uhde, accomplishes the tail-gas abatement of N2O and NO x over a single reactor using iron-containing zeolite catalysts. Two variants of the process are available depending on the tail-gas temperature. This manuscript evaluates the low-temperature abatement process (<700K), where NO x and N2O are selectively reduced by NH3 and CH4, respectively. To this end, activity tests in mixtures with N2O, O2, NO, NH3, and CH4 at different temperatures and partial pressures of reactants were carried out over steam-activated FeZSM-5. Ammonia and nitric oxide strongly inhibit the selective catalytic reduction (SCR) of N2O by CH4, shifting the temperature for N2O conversion over 700K. Controlled ammonia dosing is required for the efficient operation of the low-temperature EnviNOx® variant over the particular iron zeolite used in this study. The optimal inlet NH3 concentration is determined by the NO concentration in the tail-gas and its temperature. In the absence of NO and NH3, the CH4-SCR of N2O over FeZSM-5 is effective above 623K. Still, this process cannot retrofit a number of existing plants with lower tail-gas temperatures. [Copyright &y& Elsevier]

Abstract: Various commercially available zeolite frameworks (MFI, FER, BEA, MOR, FAU) with iron impurities ranging from 85 to 780ppm and nominal Si/Al ratios of 10–15 have been tested in direct N2O decomposition after calcination and steam treatment. Our results show a significant impact of the framework type on the catalytic activity at these high degrees of iron dilution (Fe/Al<0.01). H-beta was the most effective material in the reaction, followed by the other pentasyl-type H-ferrierite and H-ZSM-5 zeolites. [Copyright &y& Elsevier]

Selective catalytic reduction (SCR) of N2O with C3H8 over Fe-zeolites (MFI, BEA, USY) under excess oxygen is strongly inhibited by NO. The assistance of the hydrocarbon in N2O reduction vanishes at high partial NO pressures, approaching the activity of the N2O+NO system at molar NO/C3H8 ratios of 1.5–2, with N2O/C3H8=1. This effect is opposite to the NO promotion in direct N2O decomposition over these catalysts at temperatures >625 K. At lower temperatures and in the absence of NO, propane is oxydehydrogenated to propene by N2O over the catalysts. Extraframework iron sites are essential for N2O and C3H8 conversions, in particular iron in isolated positions is most effective. Propane activates on N2O-oxidized iron sites, while acidic sites play a minor role in the overall process. In the absence of NO, N2O and C3H8 conversions are coupled in temperature. Addition of NO uncouples the N2O and C3H8 conversion profiles and shifts them to higher temperature. NO inhibits N2O decomposition over iron sites by competitive adsorption at temperatures where N2O reduction with C3H8 typically starts (>525 K). At these temperatures, adsorbed NO blocks the isolated Fe-sites for N2O activation. The relatively low activity for NOx reduction to N2 (<20%) and the lower optimal operation temperature for this process (550 K) compared to N2O reduction (725 K) makes the simultaneous removal of NO and N2O with hydrocarbons unfeasible over Fe-zeolites. [Copyright &y& Elsevier]

Abstract: Microporous and mesoporous (hierarchical) ZSM-5 samples, prepared by desilication, dealumination and templating with carbon nanoparticles have been characterized by adsorbing benzene, cyclohexane and 1,3,5-trimethylbenzene (mesitylene) to probe the location, the strength and the accessibility of Brønsted acid sites. The mesoporosity introduced by the methods explored resulted in a statistical accommodation of benzene at the channel intersections. Cyclohexane showed the highest strength of physisorption in micro- and mesopores and, thus, probes the largest fractions of acid sites. Mesitylene adsorption showed that mesopores of hierarchical zeolites do not contain Brønsted acid sites. All results indicated that the mesoporosity influences only the accessibility of the acid sites in the ZSM-5 crystals mostly via shortening diffusion pathways, but not the strength of the interaction with the cyclic hydrocarbons. The enhanced transport rates observed for benzene are associated to the decreased pore length inside the mesoporous particles, rather than to changes of the pore structure or the generation of additional sites. [Copyright &y& Elsevier]

Abstract: Zeolites with various levels of micro- and mesoporosity are prepared by alkaline treatment of ZSM-5 at different temperatures. Terminal silanol groups generated at the mesopore surface during desilication are reacted with aminopropyltriethoxysilane or mercaptopropyltrimethoxysilane leading to the obtainment of hierarchical zeolites with organic-functionalized hybrid mesopores. Grafting of 3-aminopropyl and 3-thiopropyl moieties is directly evidenced by elemental analysis, nitrogen adsorption, thermogravimetry, and infrared spectroscopy, and the relative extents of surface functionalization of the purely microporous and of the mesoporous ZSM-5 zeolites are compared. Based on these results the mechanism of organosilane attachment is thought to be equivalent in both cases. Organic-functionalized mesopores exhibit good hydrothermal stability. The adsorption properties of the ZSM-5 samples both prior and post silanization are assessed for the uptake of lipase enzyme. Mesoporous zeolites show increased uptakes than the parent zeolite in all cases. This demonstrates that the mesoporosity enhances the adsorption of large guest species that cannot be accommodated within the micropores of their conventional counterparts. Immobilization on surface functionalized zeolites greatly improves the retention of lipase activity when applied in the hydrolysis of p-nitrophenylbutyrate. [Copyright &y& Elsevier]

Abstract: Several catalysts, including FeZSM-5, Co2AlO4, LaCoO3, and BaFeAl11O19, were evaluated for N2O decomposition under representative flue-gas conditions in fluidized-bed combustion (FBC). Closely related formulations proved active and stable catalysts for process-gas or tail-gas de-N2O in nitric acid plants. With this as starting point, their potential suitability for N2O abatement in stationary combustion was assessed. Tests were carried out in a fixed-bed reactor at ambient pressure and in the temperature range of 473–1123K using mixtures of N2O, O2, NO, CO, SO2, and H2O. The mixed oxide catalysts were strongly inhibited by water and sulfur dioxide and experienced fast deactivation in the simulated mixture containing all the gases. Bulk sulfate phases were detected by X-ray diffraction in the used perovskite and hexaaluminate, revealing insufficient chemical stability in the presence of sulfur and discouraging installation in the freeboard of the combustor. In great contrast, the activity of steam-activated FeZSM-5 in the model and simulated mixtures was comparable, rendering very stable performance during 30h on stream. The unique tolerance of this iron zeolite to the complex combination of feed components makes it prone to implementation after the cyclone of FBCs, where temperatures are typically 800–1100K. [Copyright &y& Elsevier]

Abstract: The combination of dealumination and desilication of ZSM-5 zeolites has been investigated in order to decouple mesoporosity creation and modification of acidity. Individually, desilication by treatment in alkaline medium leads to a spectacular mesoporosity development, without distinct modification of the acidic properties, while dealumination by steam treatment mainly impacts on the acidity of the ZSM-5 zeolite. It is shown that application of the alkaline treatment first followed by steam treatment results in MFI zeolites with independently tailored pore architecture and acidic properties, with proven hydrothermal stability under severe conditions. The independent modification of porous and acidic properties cannot be accomplished if the alkaline treatment is performed after the steam treatment, since the presence of extra-framework Al species inhibit Si extraction and related mesopore formation. A promising outlook of the successive alkaline-steam treatment is expected in catalytic applications of microporous materials. [Copyright &y& Elsevier]

Physical gas adsorption is extensively used in the characterization of micro- and mesoporous materials and is often considered as a straightforward-to-interpret technique. However, physical phenomena like the tensile strength effect, adsorbate phase transitions, and monolayer formation in combined micro- and mesoporous materials frequently lead to extra contributions in the adsorption isotherm. Models for pore size determination mostly do not account for this, and assignment to real pores leads to improper analysis of adsorption data. In this review, common pitfalls and limitations in the analysis of pore size distributions derived from adsorption isotherms of micro- and mesoporous materials are identified and discussed based on new results and examples reported in the recent literature. [Copyright &y& Elsevier]

The predominantly insulating nature of zeolites, as many classes of porous catalysts, can severely impair heat transfer and hence their performance in industrial processes. Strategies developed to engineer the thermophysical properties of technical zeolites for fixed-bed applications comprise the use of conductive secondary phases as structured catalyst supports or as inert diluents. However, the impact of integrating conductive additives into composite zeolite bodies (pellets, extrudates, or granules) has not been widely explored. Here, using a transient hot-plate technique to decouple the distinct contributions of porosity, sample hydration, and temperature, we quantify the impact of metallic (copper), ceramic (silicon carbide, aluminum nitride, boron nitride), and carbonaceous (graphite, carbon nanotubes) phases on the thermal conductivity of shaped zeolites at the body and packed-bed scales. The decisive role of particle morphology, dominating over the intrinsic conductivity of an additive, is corroborated through the three-dimensional reconstruction of data acquired by focused ion beam-scanning electron microscopy and X-ray microtomography coupled with in-situ thermographic studies. In particular, the order-of-magnitude improvement evidenced on application of graphite sheets stems from the extended paths of low thermal resistance created in the millimeter-sized catalyst ensemble. Through the identification of structure-property relations, our approach provides new insights into the rational design of composite porous materials with enhanced heat-transfer properties. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mechanochemical treatment, a versatile tool for zeolite synthesis and modification. [•] Scalable, efficient, and mechanistically-distinct to solvent-intensive routes. [•] Structural transformations classified according to the energy required. [•] Catalytic properties enhanced by mechanochemical activation. [•] Rationalization will facilitate the scale up of future zeolite technologies. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hierarchization of ZSM-22 favors hydroisomerization and limits hydrocracking [•] Hierarchical ZSM-22 has lower acidity in micropores and higher acidity in pore mouths [•] Branching occurs in pore mouths; hydrocracking in pore mouths and micropores [•] Hierarchization is a unique tool to design superior hydroisomerization catalysts [•] Pristane is a convenient model molecule for probing pore mouth catalysis [Copyright &y& Elsevier]

Abstract: The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17×4×4μm3). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120m2 g−1 of mesopore surface area, respectively, in contrast to the 62 and 5m2 g−1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification. [Copyright &y& Elsevier]

Abstract: We introduce a novel ‘spray deposition’ method as a continuous, reproducible, versatile, simple, and scalable method for the preparation of highly-dispersed, uniformly-sized, and sintering-resistant supported metal nanoparticles on porous supports. Instant drying of aqueous aerosol droplets containing the metal precursor (e.g. palladium nitrate) and the support (e.g. ferrierite zeolite) in a commercial bench-scale spray drier proves the key for maximizing the metal dispersion. In contrast, the much lower rates of solvent removal characteristic of dry and wet impregnation methods using classical drying or even freeze drying led to broad particle size distributions. The spray-dried Pd/ferrierite catalyst exhibited high alkene selectivity in the gas-phase hydrogenation of propyne. These results demonstrate that spray drying is not only a highly suited unit operation to prepare catalyst supports but also functional catalysts in one step. The method offers attractive opportunities for large-scale catalyst manufacture and as well as for preparation of well-defined systems for academic studies. [Copyright &y& Elsevier]

Abstract: Desilication of ITQ-4 (IFR code, 1-dimensional 12-MR channel system of 0.62nm×0.72nm) in aqueous NaOH solutions was practiced in order to produce the zeolite in the hierarchical form, i.e. combining the native micropores with a secondary network of intracrystalline mesopores. The parent material (Si/Al=32) was synthesized using benzylquinuclidinium as the structure-directing agent in the presence of fluoride anions. The samples were characterized by ICP-OES, XRD, TEM, N2 adsorption, and FTIR spectroscopy. In a representative treatment, the mesopore surface area of the ITQ-4 experienced a sevenfold increase (from 29 to 206m2 g−1) upon optimized NaOH post-treatment, whereas the microporosity was reduced by 70% (from 0.20 to 0.06cm3 g−1). Chemical analysis and infrared spectroscopy of adsorbed pyridine showed that after alkaline treatment the amount of Brønsted acid sites remains practically constant whereas a significant increase in Lewis acid sites was obtained. Infrared studies with adsorbed 2,4,6-collidine concluded that the accessibility of acid sites in the hierarchical zeolite increased substantially due to an increase in the available active sites at the pore mouths of the hierarchical zeolite. [Copyright &y& Elsevier]

Abstract: Desilication in alkaline medium is demonstrated as a general route to synthesize hierarchical metallosilicate zeolite crystals with preserved crystallinity. The lattice trivalent cation (not only Al3+ but also Ga3+, Fe3+, and B3+) regulates the alkaline-assisted hydrolysis of silicon towards controlled mesoporosity development. This finding widens the family of modified zeolite compositions that integrate in a single crystalline material micropores and mesopores and enables better accessibility in catalytic applications compared to the purely microporous counterparts. [Copyright &y& Elsevier]

Abstract: Zeolite beta crystals (Si/Al=35) synthesized in fluoride medium were treated in aqueous 0.2M NaOH solution for mesopore formation by selective extraction of framework silicon. A 16 parallel-batch reactor was used to study the influence of the treatment time and temperature on the physico-chemical properties of the zeolites, which were characterized by ICP-OES, XRD, N2 adsorption at 77K, SEM, TEM, DRIFTS, and in situ ATR-IR. Alkaline treatment of H-beta within the optimal window of Si/Al ratios identified for other zeolite families leads to extensive silicon extraction at mild treatment conditions. This originates substantial mesoporosity and presumably improved transport, but negatively impacts on the microporous and acidic properties of the resulting sample. Consequently, the alkaline-treated beta zeolites show lower catalytic activity in the acid-catalyzed liquid-phase benzene alkylation than the purely microporous parent material. The relatively low stability of framework aluminum in BEA, compared to MFI and MOR, is detrimental for the controlled mesopore formation by Si dissolution, since aluminum cannot optimally exert its pore-directing role. [Copyright &y& Elsevier]

Hexadecane hydrocracking was investigated over bifunctional Ru-containing micro- and mesoporous USY zeolites at 250°C under 45 bar in the presence of hydrogen in a batch reactor. A comparative study was performed using the Ru-free zeolites in the protonic form. The highest yield of jet-fuel products, C 9 - C 15 , being 26% at 71% conversion, was obtained with 2.5 wt% Ru deposited on a hierarchical micro-mesoporous USY zeolite, e.g. a material possessing secondary porosity, where a superior combination of acidity and mesoporosity was created by treatment of the parent zeolite with aqueous sodium hydroxide and tetrapropylammonium hydroxide. [ABSTRACT FROM AUTHOR]